Consanguinity

Consanguinity is basically "how close are you related?". Laws to prevent consanguinity (in other words, "don't marry your sister"), have been passed down from ancient times. The Marriage Act 1949 as amended by the Marriage (Enabling) Act 1960 establishes this list:

generally, uncles may not marry nieces, nor aunts their nephews

first cousins may wed

and a woman may marry her husband's brother, uncle or nephew if her marrige has ended by death or divorce.

Many Americans may be surprised to hear that first cousins may marry, because it is forbidden in many American states, as well as other parts of the world. The Marriage Act of 1986 (Prohibited Degrees of Relationship) makes further provision for the marriage of persons related by infinity (or marriage). The reason for these prohibitions is, firstly, public policy, and secondly, the genetic risk.

Here is the list as it appeared in a Church of England Book of Common Prayer:

Statutes of 1907 and 1921 made an exception at a. above, allowing people to marry the spouse of the brother or sister, if that brother or sister had died. Some further exceptions were made in 1931, 1949 and 1986 so that, for example, a man was allowed to marry his deceased wife's niece, aunt or widowed mother.